The present invention relates to a light-emitting device, termed a white LED, comprising a semiconductor light-emitting element light source, and a fluorescent material that receives output light therefrom and that emits fluorescent light of a different wavelength than this output light, the light from the light-emitting element and the light from the fluorescent material being combined to produce white light.
A number of white LEDsxe2x80x94light-emitting devices that use semiconductor lightemitting elements to produce white lightxe2x80x94have been proposed to date. The use of semiconductor light-emitting elements affords relatively intense light with low electrical power consumption. Further, unlike incandescent bulbs or fluorescent lights, such devices to not radiate heat, and do not experience problems such as deterioration with time or burning out. Applications for such devices are thus expanding rapidly. Japanese Patent No. 2927279 discloses a technique for producing a white LED using a semiconductor light-emitting element. This patent teaches combining blue light (output by a gallium nitride semiconductor element) with yellow light having a broad spectrum component (output by a YAG fluorescent material which is excited by blue light output) to produce white light. In this prior art, the white LED is produced by arranging the semiconductor element on a substrate and encapsulating it in a transparent resin containing YAG fluorescent material.
Light sources that use gallium nitride semiconductor elements have longer life than the incandescent bulbs and fluorescent lights currently used as light sources for illumination, and can be used for up to about 10 years.
However, the light-emitting devices disclosed to date employ a resin protective layer (molding) to protect the light-emitting diode, and this poses a number of problems. For example, where the protective layer is composed of a resin, water can penetrate in the course of service over several years, impairing operation of the light-emitting diode; or where the output light of the light-emitting diode is ultraviolet, the ultraviolet can eventually cause discoloration, reducing ability to transmit output light from the light-emitting diode, and substantially impairing light-emitting diode performance.
In another aspect, the YAG fluorescent material disclosed in the prior art emits a broad spectrum of light centered around yellow. However, color rendering is poor, as noted. With the aim of improving color rendering, the Applicant has proposed a light-emitting device that combines two fluorescent materials, a green-and a red-light emitting material. However, these fluorescent materials have poor moisture resistance, so moisture countermeasures are crucial. However, light-emitting device designs proposed to date do not afford adequate moisture permeability.
In view of the problems with resin light-emitting diode protective layers, Unexamined Patent Application (Kokai) 11-251640 and Unexamined Patent Application (Kokai) 11-204838 disclose protective layers for protecting light-emitting diodes. These Applications were proposed in view of the drawbacks of the resin light-emitting diode protective layer disclosed in the aforementioned patent, namely, susceptibility to permeation by moisturexe2x80x94i.e., poor environmental resistancexe2x80x94and discoloration with massive exposure to ultravioletxe2x80x94i.e., poor ultraviolet resistancexe2x80x94, resulting in diminished transparence and impaired characteristics as a light-emitting diode, and teach encapsulating the light-emitting diode with sol-gel glass rather than with a resin protective layer.
However, the light-emitting devices disclosed in Unexamined Patent Application (Kokai) 11-251640 and Unexamined Patent Application (Kokai) 11-204838 have the following problems.
Where wire bonding is employed to provide reliable electrical contact of the light-emitting diode, encapsulating the light-emitting diode with sol-gel glass using the methods disclosed in the above publications poses the following potential problems.
Where wire bonding is employed to provide reliable electrical contact of the light-emitting diode with an outside power supply, the wires from the light-emitting diode must pass through both glass and epoxy in order to connect with the leads outside the light-emitting diode. However, as the glass and epoxy resin have different qualities, such as coefficients of thermal expansion and hygroscopicity, significant stresses may be created within wires at the glass-epoxy interface, possibly severing the wires. Where sol-gel glass is used, volume shrinks by about 30% during hardening, so breakage resulting from stresses created in the wire can occur during molding as well. Thus, where wire bonding is employed to provide reliable conduction paths, differences in physical properties at the glass layer/epoxy cap interface can result in the problem of wire breakage.
In short, with light-emitting devices employing semiconductor light-emitting elements, while the semiconductor elements per se have high reliability and extended life, the packaging used to protect the semiconductor light-emitting element and ensure electrical contact with an outside power supply tends to experience problems in terms of reliability.
While it would be possible to address this problem by using a thicker glass layer to create the interface, it is difficult to produce thick glass that is free of cracks.
With the foregoing in view, it is an object of the present invention to provide highly reliable packaging for semiconductor light-emitting elements, and to thereby provide a light-emitting device employing a semiconductor light-emitting element that offers sustained high performance and extended service life.
This object herein is achieved through a semiconductor light-emitting device wherein a semiconductor light-emitting element flip-chip is electrically interconnected to terminals on a substrate, said device comprising: a light-emitting element consisting of a gallium nitride semiconductor element; and a glass layer arranged on the path of the light output by said light-emitting element and containing a fluorescent material for receiving said output light and producing converted light converted to a wavelength different from that of said output light; wherein said emitted light and said converted light are used to produce essentially white light.
In preferred practice, the substrate will be a printed board.
In a preferred embodiment, the fluorescent material will consist of two sulfur-containing compositions, each fluorescent material producing converted light of a different wavelength.
One of the two fluorescent materials may be SrS:Eu2+ that emits red fluorescent light, with the other being (Sr, Ba, Ca)Ga2S4:Eu2+ that emits green fluorescent light.
The green fluorescent material may consist of SrGa2S4:Eu2+. In a preferred embodiment, the glass layer containing the fluorescent material will have a thickness of 100 xcexcm or less.
In a preferred embodiment, the glass layer will consist of SiO2 containing at least one compound selected from the group consisting of PbO, Ga2O3, Bi2O3, CdO, ZnO, BaO, and Al2O3; or of SiO2 substantially devoid thereof.
Glass layer composition may be manipulated by including compounds selected from PbO, Ga2O3, Bi2O3, CdO, ZnO, BaO, and Al2O3. The reason for doing this is as follows. Reflection occurs at the interface of the light-emitting element and the surrounding glass layer or other packaging material. The proportion of total reflection occurring is higher the greater the difference in refractive index. Total reflection results in light bouncing back and forth within the package so that the efficiency of light emission to the outside declines. Accordingly, it is desirable to minimize total reflection at interfaces through which light from the light-emitting element passes, so as to achieve efficient transmission of light from the light-emitting element to the air. In order to achieve this it is necessary to minimize refractive index differential at each interface. Where a light-emitting element structure comprises two layersxe2x80x94a light-emitting element and a glass layerxe2x80x94two interfaces are present, one of the light-emitting element with the glass layer and one of the glass layer with the air layer, so in order to minimize reflection of light from the light-emitting element, it is desirable to minimize refractive index differential at each of the two interfaces.
Accordingly, the refractive index of the glass layer preferably lies between the refractive indices of air and that of the light-emitting element.
By providing the surface of the element with the aforementioned oxide having a refractive index lying between the refractive index of air and the refractive index of the light-emitting element, and using this as a non-reflective coating to increase the efficiency of light emission to the outside, light output can be improved on the order of several ten percent. The advantages of using glass coating rather than encapsulation with epoxy are that:
1) environmental resistance is improved; 2) operation at high temperatures becomes possible; and 3) materials affording higher efficiency of light emission can be selected. The refractive indices of epoxies in no event exceed about 1.6. The refractive index of air is 1.0, and the refractive indices of compound semiconductors used in light-emitting diodes range from about 3.4 to 1.8. The refractive index of the SiO2 of the glass layer is about 1.5, but where the SiO2 contains a transparent compound such as PbO, Ga2O3, Bi2O3, CdO, ZnO, BaO, or Al2O3, the refractive index can be increased to about 1.5 to 2.5. Thus, by including a transparent compound such as PbO, Ga2O3, Bi2O3, CdO, ZnO, BaO, or Al2O3 in SiO2, the refractive index of the glass layer can be controlled to a desired level so as to maximize the efficiency of light emission from the light-emitting element.
Where an additional epoxy layer is provided to the outside of the glass layer, the refractive index at the interface of the glass layer with the epoxy layer is considered as well. That is, the refractive index of the glass layer is suitably controlled by manipulating the amount of transparent compound contained therein so as to minimize refractive index at the respective interfaces going from the light-emitting element to the glass layer, epoxy layer, and air. Specifically, the changes in refractive index going from the light-emitting element to the glass layer, epoxy layer, and air will constitute a geometrical series.
As noted, the invention provides a light-emitting device employing a combination of blue light from a gallium nitride LED and light of converted color from a fluorescent material to emit essentially white light. The white LED herein, used in conjunction with a blue LED, serves as a highly reliable, extended life light source for illumination. White LEDs employing high-output blue LEDs as excitation sources can replace fluorescent lamps, incandescent bulbs, or the like. By means of a reliable packaging process, the light-emitting device herein provides a light-emitting device that offers high performance, notwithstanding the use of a combination of fluorescent materials having poor environmental resistance but also affording enhanced color reproduction.